nfsd4: fix minor memory leak

There's no need to allocate this cred more than once.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
index 8fa412c..4bc22c7 100644
--- a/fs/nfsd/nfs4callback.c
+++ b/fs/nfsd/nfs4callback.c
@@ -525,6 +525,8 @@
 
 int set_callback_cred(void)
 {
+	if (callback_cred)
+		return 0;
 	callback_cred = rpc_lookup_machine_cred();
 	if (!callback_cred)
 		return -ENOMEM;